The thermoplastic processability as one of the main advantages of DV when compared to conventional rubber requires a prediction of the effect of cooling rate as one major processing parameter on the final structure and properties of the part. This is important especially for the production of thin walled parts and profiles as used in automotive industry especially when
combined with other thermoplastic materials in multi – component injection molding where high temperature gradients are applied during processing.
The aim of this work was the description of the cooling process during DV processing, the structure formation in dependence on cooling rate, and finally the relationship between technological process conditions and the structure and properties of DV. An intensive cooling method where distinct cooling rates can be applied during the solidification of the material has been developed by Piccarolo et al. /3/. This method was already used to study the influence of fast cooling on the crystallization behavior of different semicrystalline polymers and was applied in this work on DV. The experimental investigations were performed on three DV prepared by dynamic vulcanization in a twin screw extruder, changing the type of elastomer, the type of crosslinking agent, and the amount of thermoplast content in order to observe their role in the development of structure and properties parallel influenced by cooling rates.
Morphological investigations of the pure components showed, that only iPP was influenced by cooling rates whereas the morphology of the copolymers EOC and EPDM showed to be uninfluenced by this processing parameter. The WAXD pattern of EOC and EPDM showed only a single reflection remaining the same for the samples solidified with different cooling rates. The WAXD pattern of iPP showed for slow cooling rates the typical α – monoclinic pattern, which was replaced by the mesomorphic pattern when going to higher cooling rates.
In order to receive a numerical value of the amount of phases present in the material when cooled with different cooling rates a deconvolution technique has been applied as used by Martorana /79/. The phase content distribution of pure iPP vs. cooling rates diagrams showed at low cooling rates an equal amount of amorphous and α – monoclinic phase, both around 50%. Between 20 and 80 K/s the amount of α – monoclinic phase dropped to almost zero being at the same time replaced by the mesomorphic phase. This zone was named transition zone. At high cooling rates the values of all phase contents remained at stable levels with the amorphous phase taking up slightly more content than the mesomorphic phase.
The WAXD pattern of DV appeared as superposition of the copolymer and the iPP patterns.
A visible change in the pattern with cooling rates was assigned only to the iPP pattern. The phase fractions vs. cooling rates of the iPP matrix of the DV showed a similar dependence on cooling rates in comparison to pure iPP. The mesomorphic phase replaced the α – monoclinic phase completely and the amorphous phase remained stable implying that the formation of mesomorphic phase at high cooling rates is favored with respect to the amorphous phase possibly do to the presence of the rubber phase. The drop of α – monoclinic phase in the iPP matrix of a DV is shifted to higher cooling rates, which was explained by the presence of crosslinked copolymer forming boundary layers giving rise to heterogeneous crystallization.
The density values of iPP and the DV depended on cooling rates in almost the same manner.
Three zones (alpha, transition and mesomorphic zone) were indicated. In comparison to the density change of the bulk iPP the density change of the iPP matrix in DV it was found that the transition zone is shifted to higher cooling rates and wider. In other words, the iPP matrix of a DV which has been solidified at a cooling rate of 50 K/s appeared still α-crystalline while the bulk iPP solidified at the same cooling rate had already formed the mesomorphic phase.
This implied the presence of a more stable α-crystalline form in DV. Even though the formation of perfect crystals was hindered by the rubber phase more stable crystals were formed due to enhanced heterogeneous crystallization on the phase boundaries between rubber and iPP. In the presence of additionally nucleating structures, as SnCl2 in the PP/EPDM 30/70r, the transition zone extended even further to higher cooling rates.
AFM micrographs showed for PP/EPDM 30/70p the typical particle-matrix-morphology of DV. The size of the rubber particles seemed to be divided into larger particles of approx. 1µm and smaller particles of approx. 0.05 µm. The matrix of the slowly cooled samples showed cross - hatched lamellae surrounding the rubber particles. The phase image of the fast cooled sample showed a much lower contrast between the iPP matrix and the EPDM rubber phase indicating a lower hardness. In fact no crosshatched lamella were observed only very few lamellar structures of smaller dimensions.
Investigations of the thermal behavior of iPP and the DV showed a strong influence of the changed morphology due to cooling rate on thermal transitions. DSC scans of iPP showed with increasing cooling rates the formation of an exothermal area starting before the onset of endothermal melting. This small exothermal local maximum was assigned to reorganization processes of the mesomorphic phase taking place at elevated temperature. A peak of tan delta values from DMTA measurements of the fast cooled iPP samples, named meso-transition, different from the α-transition peak observed for the slowly cooled samples, supported this interpretation of melting of the mesomorphic phase and subsequent recrystallization. Similar results have been found for the DV although this reorganization peak in the DMTA data was partly superimposed by the softening of the copolymer phase. Tan delta peaks obtained by DMTA measurements of the DV showed a glass transition of EPDM around –30 °C and – 40°C in case of the EOC, being independent of cooling rates. As observed by DMTA measurements the β-transition or glass transition of the pure iPP as well as of the iPP matrix of the DV increased with growing cooling rates from 15 °C to 25 °C. This was explained by a hindered segmental movement of the amorphous phase in the presence of immersed mesomorphic phase consistent of small, organized forms with low order. The slowly crystallized iPP instead contained distinct crystalline phases well separated from the amorphous regions allowing the movement of the amorphous phase to a greater extend.
DMTA measurements showed a strong dependence of storage modulus and cooling rates. The storage modulus of fast cooled iPP at room temperature was only half of the value of the slowly cooled sample. The storage moduli of the DV at room temperature appeared at a much lower level of only 3 % of the value measured for the pure iPP due to their morphology.
Nevertheless, the storage moduli of the fast cooled DV amounted only to values of approx. 65
% of the storage moduli of the slowly cooled samples.
The dependence of storage moduli on cooling rates implied a strong connection between the thermal history and the mechanical behavior of the DV. Miniature tensile test showed an influence of cooling rates on the stress - strain behavior of iPP and DV. Generally lower stress values were reached at high cooling rates. The DV showed a more elastomeric behavior exhibiting lower tensile strength values than iPP. The addition of thermoplast to the DV resulted in higher tensile strength values, especially for the samples cooled at slow cooling rates. This supported the assumption that the tensile strength values depend directly on the amount of crystallinity. The stress values of samples solidified at high cooling rates did not show such a high influence of thermoplast content and reached the same level of stress at higher strain rates. Enhancing the chemical network density of the rubber phase by means of a higher amount of crosslinking agent increased the tensile strength values equally, i.e. also the fast cooled samples reached higher values except the blends, where only co-continuos morphology is present.
Rheoptical measurements on PP/EPDM 30/70r revealed that the iPP matrix showed a higher degree of orientation when solidified at higher cooling rates than at low cooling rates. This
was explained by heterogeneous deformation in the thermoplastic matrix where the local shear stress overcome a critical values being lower for the mesomorphic phase than for the α -monoclinic phase. Also the EPDM phase of the fast cooled samples was oriented to a much higher degree than the slowly cooled sample. The critical shear stress of the deformation of the mesomorphic phase of iPP is much lower than of the α-phase. The plastic flow is more enhanced allowing also the orientation of the distributed elastomer phase.
DV showed less reversibility when solidified at high cooling rates. The higher values at high cooling rates implied a higher amount of plastic deformation of the iPP matrix due to the increased amount of mesomorphic phase. The work of irreversible deformation was lower at high cooling rates, which means the work necessary to deform the mesomorphic phase plastically is lower than for the α-monoclinic phase. The reversible work of deformation did not depend on cooling but was influenced by the chemical crosslinking density of the elastomer phase. These observations were supported by the rheooptical measurements of the orientation of the individual phases. A two - network-model was used to explain the deformation behavior of DV. One network consists of physical network points in shape of crystalline structures, being therefore dependent on the type and amount of crystalline form.
The second network is formed by chemical stable network points. Both networks contribute to the deformation and reversibility of a DV.
The microhardness dependence on cooling rates was similar to the dependence of density vs.
cooling rate. The microhardness of DV showed a negative exponential dependence on amount of thermoplast content. Based on the data of polymorphic phase content from WAXD microhardness values have been determined for each phase and subsequently summed up to obtain the total microhardness values. These values placed in the exponential equation to describe the influence of thermoplast content gave the possibility to predict microhardness values for a range of cooling rates for different amounts of rubber.
The method of intensive cooling from the melt as developed by Piccarolo et al. /3/ proved to be applicable also for DV in order to determine the influence of cooling rate as one parameter of thermoplastic processing on the crystallization behavior and subsequent morphology development. The rubber phase of the DV used in this study was not influenced by the cooling rate. Nevertheless this cannot be excluded for different rubbers with a higher degree of crystallinity. Further investigations must be performed to clear this aspect. The effect of cooling rates on DV was originated in the iPP matrix and its interaction with the rubber phase.
The total changes of properties with cooling rate were much lower in DV than in pure iPP due to the small thermoplast content. However, there is a considerable influence of cooling rate on the mechanical properties. This study gives the possibility to predict property changes in DV according to the applied cooling rates during processing, as shown for a example of bi-component molding of iPP and DV. In order to receive similar specific volume changes in order to lower warpage, lower cooling rates should be applied on the iPP cavity wall with respect to the DV cavity wall. Finally it must be made clear that cooling rates are one of several parameters influencing the final part structure and properties, such as pressure and flow rate. The influence of these parameters in connection with cooling rates on structure and properties of a DV must be investigated in the future.